1. Field of the Invention
The present invention pertains to a method and apparatus for enhancing survivability of a liquid containing structure subjected to penetration by high velocity exploding projectiles. With greater particularity, the present invention pertains to a fuel tank construction for use in a military aircraft. With greatest particularity, the present invention pertains to a military aircraft fuel tank construction using composite materials that also utilizes selective placement of low density, fluid-displacing material. This low density, fluid-displacing material creates a low shock impedance region which decouples protected fuel tank structure from the destructive effects of combined far and near field loading caused by a shock wave and hydrodynamic ram effect accompanying tank penetration and detonation of a high velocity projectile.
2. Description of the Related Art
Aircraft powered by conventional propulsion means typically utilize liquid hydrocarbon fuels such as kerosene or other blended aviation products. Aircraft usually carry their fuel supply in tanks integrated within the aircraft structure, either in the wing or fuselage. Modern aircraft construction has moved beyond all aluminum structure towards structures utilizing greater amounts of composite materials. Such materials provide greater strength-to-weight ratios than metal, and they benefit military aircraft by offering a lower radar return signature. Composite structures, when loaded to the point of failure, tend to be more brittle and to fail more abruptly and less gracefully than all aluminum structures. Military aircraft manufactured using composite materials, typically a carbon fiber or other fiber matrix impregnated with synthetic resin, present demanding survivability challenges to aircraft designers.
Military aircraft fuel tanks utilizing composite materials can be particularly vulnerable to projectiles that penetrate the liquid fuel containment volume and produce a shock wave from their high velocity (hydraulic ram effect), but more importantly produce a shock wave when an explosive charge contained within the projectile, detonates. High-speed projectiles may originate from enemy military aircraft cannon fire or from ground fire. Such projectiles can enter aircraft structure at supersonic velocity. When high-speed exploding projectiles encounter a fluid that is more dense than air, such as jet engine fuel, they can produce a shock wave that can have destructive effects upon adjacent aircraft fuel tank structure. These destructive effects can be catastrophic in an aircraft using composite materials if the fuel tank wall is caused to separate from internal spars and other stiffeners.
Many prior attempts to enhance survivability of aircraft fuel tankage are represented by U.S. Pat. No. 3,787,279 to Winchester for Shock and Fire Attenuating Fuel Tank. This United States patent, assigned to the United States Navy, uses a continuous layer of fiber reinforced foam material as a liner inside a metallic tank to reduce damage caused by hydraulic ram pressure effects. This design provides a continuous layer of foam material covering the inner surface of the fuel tank wall.
Another approach, represented by U.S. Pat. No. 4,172,573 to Moore, et al., for Fuel Tank, provides a crash-worthy fuel tank which has built-in unfilled space or energy-absorbing material which can attenuate some internal pressure resulting from external forces acting on the tank, so that the internal fuel pressure does not cause fuel tank rupture. This approach utilizes internal bulkheads which separate an unfilled space or energy absorbing material from the contained fluid or fuel volume. In this manner, internal fuel pressure resulting from external compression of the tank walls, acts on the unfilled or energy-absorbing material volumes contained within the tank rather than acting directly on the external tank walls that may lead to tank wall rupture. This design provides a margin of safety which enhances fuel tank survivability.
Yet another more specialized approach, shown in U.S. Pat. No. 4,886,225 to Bates for Inflatable Fuel Tank Buffer, provides an inflatable bladder on the interior surface of an integral fuel tank, between the wall of an aircraft inlet duct and the liquid fuel, to reduce the potential damage caused by hydraulic ram effect from high speed projectile penetration of the tank wall to thereby reduce the size of the projectile exit hole and reduce the volume of leaking fuel that may flow into the engine inlet and choke the engine.
Another approach uses resilient bladders filled with an inert gas to continuously line the walls of the fuel tank to protect fuel in the tank from explosion or leakage in the event that the tank is punctured by a ballistic projectile. This approach is described in U.S. Pat. No. 4,925,057 to Childress, et al., for Fuel Tank Having Ballistic Protection Bladder. In this embodiment, a plurality of bladders line the entire internal surface of the fuel tank.
All of these approaches to the problem of enhanced fuel tank survivability, provide continuous protection to the fuel tank walls because of random uncertainty as to the exact location and direction of a penetrating projectile, and assumed equal vulnerability at all locations. This approach gives up fuel tank volume in return for enhanced fuel tank survivability. Such construction is often recommended in metallic fuel tanks where the tank is equally vulnerable to penetrating projectiles over its entire surface, but such techniques may be wasteful in composite fuel tanks, especially for fighter type aircraft where fuel capacity is at a minimum anyway, and where some portions of the tank may be more or less robust than others.
The conventional approach to mitigating structural damage is to add more structure or toughen the structure with different materials or constructions. This approach generally means adding a cost and weight penalty to a system and does not address the control and propagation of the damage resulting from a passing shock wave. Often the solutions found are point designs that cannot be applied to a broad cross-section of various structural designs. Other approaches, as illustrated above, include placing foam liners or inerting bladders into the fuel cells. In general, these approaches can be effective for fire suppression, but have very limited effectiveness against hydraulic ram pressure effects. Modern high performance systems cannot tolerate the loss of fuel capacity that these approaches require.
Although continuous foam liners may be partially effective in attenuating damage caused by hydraulic ram pressure, the use of a continuous foam layer over the entire inner surface of the tank exacts a sometimes unacceptable penalty in terms of reduced fuel tank capacity, especially in a fighter type aircraft having a relatively thin wing integral fuel tank. Additional penalties of increased dry weight and cost contribute to make it a less than optimum solution. A continuous layer of foam is not effective at selectively tailoring the shock wave response profile of the structure as does the present invention since the foam layer is of equal thickness on all surfaces, and again this solution exacts a major penalty in terms of reduced fuel volume that the tank is capable of containing.
Fuel tanks manufactured from composite materials may exhibit greater strength and at the same time greater vulnerability in different places, depending upon the fuel tank design details, than would a metallic tank. For such a composite fuel tank it may not be necessary to provide protection to areas which are less vulnerable than others. In fact, such protection should be concentrated in those areas where structural damage is most likely to lead to catastrophic failure and destruction of aircraft structural integrity. In a composite structure fuel tank, areas of catastrophic vulnerability typically exist where the external fuel tank wall is joined to internal spars and stiffeners. At this junction, separation can lead to loss of structural integrity of the surrounding aircraft structure, usually wing structure, resulting in loss of aircraft control.